also Sprint's maintenance window is 1am - 5am when doing anything service/network/customer impacting, as an FYI.

That's not completely true. It depends on what you're doing. I have "just a little experience" in the field.
As for AT&T's problem yesterday, there's only about 3 things that could have happened and 2 of them are highly unlikely.
Before we get into a disagreement about Sprint having the fewest outages, remember that both AT&T and Verizon have to report all FCC outages, not just cellular. Sprint doesn't serve any landline based customers, but AT&T and Verizon do. I don't remember the exact critera for a reportable outage, but part of it is anything that effects more than 30K subscribers. Sprint does serve some customers for data and such via fiber, but the likelihood for that to fail to the point of an outage reportable to the FCC is pretty slim.